Manipulating Quantum Systems An Assessment of Atomic, Molecular, and Optical Physics in the United States (2020) / Chapter Skim
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3 Emerging Phenomena from Few- to Many-Body Systems
3 Emerging Phenomena from Few- to Many-Body Systems
Pages 65-103
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From page 65... ...
In this fascinating regime, quantum phenomena that challenge our intuition become commonplace. This chapter focuses on describing the advances in experimental control that have produced realizations of a broad range of quantum mechanical phenomena, for as few as two atoms, up to many-particle systems with thousands or even millions of atoms or molecules.
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From page 66... ...
NOTE: BEC, Bose Einstein condensate; DFG, degenerate Fermi gases. SOURCE: Chris Greene.
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From page 67... ...
In many cases, these phenomena are fascinating to study in their own right, while in other cases their value derives from the ability to simulate novel phenomena in condensed-matter and topological physics with an unprecedented level of control. Recent years in the field of ultracold quantum gases have seen remarkable achievements.
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From page 68... ...
As discussed below in the section "Analog Quantum Simulation of Strongly Correlated Quantum Many-Body Systems," this type of quantum simulation has the potential to simulate the behavior of systems that are currently beyond our ability to treat reliably using existing theoretical methods or computers. Such applications are at the forefront of current quantum information applications.
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From page 69... ...
For instance, the magnetic controllability of interactions enabled the first observation in 2006 of the intriguing, counterintuitive quantum states known as "universal 3-atom Efimov states," and many subsequent experiments have similarly used magnetically tuned resonances to observe Efimov trimers in both homonuclear and heteronuclear cases. The word "universal," used in this context, refers to the fact that any three-particle system with limited range but strong interactions (i.e., large scattering lengths)
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From page 70... ...
Another surpris ing feature of this odd corner of the quantum mechanical world of three weakly attracting particles is that there is one condition (infinite two-body scattering length) for which there are predicted to be an infinite number of these so-called Efimov states even though no stable dimer states exist.
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From page 71... ...
and first excited state (red and black, the Efimov state, measured two different ways) of three helium atoms, compared with current theoretical predictions (purple)
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From page 72... ...
FIGURE 3.2.1 Formation of a Fermi gas, one atom at a time.
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From page 73... ...
Cooling and trapping of ultracold molecular ions is another topic that has advanced rapidly in recent years, and this is being pursued with a number of different long-term goals. Some of them are focused on precision physics tests of fundamental symmetries of the universe, including measurements of the electron electric dipole moment or of the electron to proton mass ratio to explore the possibility of a time-dependent ratio.
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From page 74... ...
Photoionization of laser-cooled clouds of multiple atomic species yields more complex, strongly coupled plasmas, which are being studied to investigate phase separation and interspe cies thermalization. Ultracold neutral plasmas are also formed by photoionizing molecules that DEVELOPMENTS WITH ATOMIC DEGENERATE QUANTUM GASES Once atoms have been cooled down to temperatures in the range 1-100 nanoK, new states or phases of matter have been predicted to occur, and by now many novel phases have indeed been created and observed.
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From page 75... ...
While some significant theoretical understanding of this very challenging regime of many-particle physics has emerged, challenging puzzles remain, and a great deal more needs to be unraveled in future theoretical and experimental studies. See further discussion below in the section "Unitary Quantum Gases." One major development in the field of ultracold quantum gas physics has been a significant expansion in the types of atoms that can be controlled and brought into quantum degeneracy.
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From page 76... ...
Polaron physics in ul tracold atomic systems mimics analogous physics in a number of condensed-matter systems, but with far greater control over the interactions between the impurity and the majority sea of atoms. The committee elaborates on this polaron physics below in the section "Polaron Physics with Ultracold Atoms." Unitary Quantum Gases One of the most interesting topics being studied by experiments and theory 15 years ago was the two-component degenerate Fermi gas, as the atom-atom scat tering length a was tuned through the unitary limit, a → infinity.
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From page 77... ...
While no ground-state atom possesses an electric dipole moment, they frequently do possess a magnetic dipole moment. Polar molecules, by definition, possess a permanent electric dipole moment, as do many Rydberg atoms or molecules (see the section below, "Quantum Simulation with Dipolar Interactions")
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From page 78... ...
For the system of magnetic dipolar atoms (erbium at oms, shown in the bottom panel) , there are attractive dipole-dipole interactions that can support a regime for the mean-field energy to be attractive, and the size of self-bound droplets stays constant as time increases, although the density gradually decays as a result of three-body loss processes.
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From page 79... ...
Here, novel macroscopic quantum phases of matter emerge, revealing an astonishing universality that connects the behavior of dense quantum fluids (like superfluid helium) to dilute dipolar gases such as those discussed above, through fundamental properties of quantum mechanics.
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From page 80... ...
Thanks to ad vances in both experiment and theory, the energy spectrum of polarons is now widely understood in many basic situations, and for large ranges of the interaction strength. It is remarkable, and worth noting, that research on quantum gases has stimulated the discovery of new polaronic quasiparticles in more traditional solid state semiconductor materials.
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From page 81... ...
The effective mass is a perturbative approach that assumes that the only effect of the medium on the impurity is to increase its apparent mass. This increase in effective mass represents the kinetic energy stored in the atoms of the medium as they "get out of the way" of the moving impurity.
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From page 82... ...
In a cross disciplinary sense, it will be very fruitful to further build on the analogies of AMO based systems with real condensed-matter systems. MANY-BODY SYSTEMS WITH ULTRACOLD MOLECULES In comparison with atoms, molecules have a much more complex level structure and many more quantum degrees of freedom, and their control is a major challenge in achieving ultracold molecular quantum gases.
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From page 83... ...
This approach yields exceptionally low temperatures; it has enabled trapping of molecular arrays in optical lattices, and production of a quantum-degenerate gas of polar molecules. The range of molecular species that can be assembled in this way is limited, but sufficient for some envisioned applications.
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From page 84... ...
Hence, even when molecules are separated by distances on the order of the wavelength of visible light, this coupling between molecules may dominate the dynamics of the system. This means that many-body systems of polar molecules in optical lattices or tweezer ar rays can be engineered to become highly entangled, and to remain so for long times.
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From page 85... ...
These and other experiments have demonstrated long coherence times of molecular spin superpositions, and improved rotational state coherence by better optimizing the optical-lattice parameters. These constitute early, but extremely promising first steps toward using ultracold polar molecules as a system for interesting, novel types of quantum simulations, as discussed below.
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From page 86... ...
Moreover, it opens the door toward realizing strongly correlated quantum phases in a laboratory setting, by tuning of the Hubbard parameters via ex ternal fields. Hubbard models built from ultracold atoms and molecules in an optical lattice provide us with a paradigmatic example of an "analog quantum simulator," where a quantum many-body system can be realized in a controlled setting.
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From page 87... ...
. Quantum Simulation of Fermi-Hubbard Models in Various Spatial Dimensions As described above, quantum simulation of the Hubbard model with ultracold fermionic atoms in optical lattices is a prime example of emerging quantum systems.
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From page 88... ...
Ultracold fermionic atoms in an optical lattice form a model system for simulating strongly correlated electron systems in materials.
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From page 89... ...
There is a rich literature of theoretical proposals for how to engineer strongly correlated quantum many-body systems with ultracold magnetic atoms and ultracold polar molecules, and these phenomena are now beginning to be explored in the laboratory, both with highly magnetic atoms in optical lattices (such
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From page 90... ...
. The unique feature of polar molecules in designing quantum simulators is, first of all, their strong electric dipoles and corresponding strong dipolar interactions.
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From page 91... ...
If the atom has, say, five distinct internal spin states, one can arrange for hopping transitions to occur among the neighboring spin states in such a way that the atom appears to be moving in quasi-2D optical lattice strip of width 5. Quantum gas systems with a synthetic dimension have been used to explore a version of the famous quantum Hall effect in condensed-matter systems, except that in the cold atom version of this effect, the motion of electrically neutral atoms mimics the behavior of charged electrons in a strong magnetic field.
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From page 92... ...
Theoretical investigations of correlated quantum many-body systems have pro ceeded at an astonishing pace. However, experimental progress on such correlated systems embodied in atomic quantum gases has been hampered by the extremely low entropy (essentially temperature)
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From page 93... ...
Today, in parallel to the intense effort that is dedicated to the search for novel topological materials, a substantial research activity concerns the realization of topological states using engineered quantum systems, such as ultracold gases trapped in optical lattices. Indeed, these engineered systems could offer the possibility of accessing a wide variety of topological phenomena in a highly controllable environment, an ideal setting to explore and manipulate topological matter in regimes that are hardly accessible in the solid state.
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From page 94... ...
This would be an important step toward exploration of new sci ence, and such phenomena are also believed to hold promise for technologically important applications in quantum information processing. The realization of strongly correlated topological states of matter using ultra cold atomic gases is deeply connected to another active field of research: the quan tum simulation of lattice gauge theories, for probing fundamental particle physics.
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From page 95... ...
communities. The report will return to quantum simulation of dynamical gauge fields -- that is, where the gauge field becomes a dynamical variable -- in Chapter 4.
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From page 96... ...
Second, within a well-defined thermodynamic phase, the system's ordering should be robust against a wide range of perturbations of both the initial state and the equations of motion. These topics are currently being explored in a wide variety of AMO systems, including ultracold gases in a quantum simulator setting and trapped-ion analog quantum simulators (see also Chapter 4)
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From page 97... ...
These questions are also deeply linked to emerging quantum technologies, whose protocols involve dynamical control of quantum many-body systems in one way or another. Analog AMO quantum simulators can provide experimental insight into these fundamental and challenging problems (see Figure 3.9)
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From page 98... ...
However, later experiments using the newly developed quantum gas microscope platform, with access to individual components of the many-body system, extracted quantum entropies, or explored localization in two dimensions, in a regime where no existing theoretical method is able to predict the system behavior. The latter experiments are a prototypical example where AMO quantum simulators provide truly new insight into a difficult quantum many-body problem, and thus offer a practical quantum advantage over classical simulations.
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From page 99... ...
On the topic of manybody localization and far from equilibrium dynamics in general, these synthetic many-body systems provide the only available, or the most advanced, experimental platform to explore the dynamic behavior of quantum matter. Open System Quantum Simulation: Photonic Crystal Waveguides The quantum simulation the committee has discussed so far refers to engineering many-body Hamiltonians and dynamics of isolated systems with ultracold atoms and molecules.
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From page 100... ...
Such systems are emerging as leading candidates for realization of quantum networks, discussed in Chapter 4. From Analog Quantum Simulation to Quantum Information Science Analog quantum simulators, as mappings of quantum many-body Hamil tonians of interest to the "natural" Hamiltonians provided by a particular AMO platform, are, of course, not restricted to atoms and molecules in optical lattices
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From page 101... ...
In the few-body limit, it is of continuing interest to identify the scope of universality in quantum states, for their own intellectual interest, for their connections with many-body physics, and for the potential to add new types of controllability at both the few-body and many-body level. Ultracold molecules are starting to form a more diversified research platform, where molecular quantum gases promise to tackle a rich set of many-body phenomena, chemically interesting cold molecules will provide new insights to fundamental reaction processes, and molecules chosen for specific precision measurement targets are being brought under increasingly sophisticated levels of quantum control.
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From page 102... ...
Last, the "natural" fermions provided by fermionic species in atomic quantum simulation also offer unique opportunities, as discussed in Chapter 4 in context of variational quantum simulation with Fermi Hubbard models as the basis of programmable quantum simulators. Finding: Few-body physics continues to be of continuing interest to identify and test the scope of quantum universality, for its intrinsic intellectual interest, its connections with many-body physics, and to strengthen the controllability of both few-body and many-body quantum systems.
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From page 103... ...
Recommendation: The atomic, molecular, and optical science community should aggressively pursue, and federal agencies should support, the de velopment of enhanced control of cold atoms and molecules, which is the foundational work for future advances in quantum information processing, precision measurement, and many-body physics. Finding: Quantum gases of atoms and molecules enable controlled exploration of equilibrium and non-equilibrium many-body physics and the generation and manipulation of entangled states applicable to quantum information pro cessing and quantum metrology, and further developing our understanding of deep questions such as the nature of thermalization, many-body localization, and stable quantum matter away from equilibrium.
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